SOLE EDITION of the poet and antiquary John Leland's defense of the historical reality of King Arthur against the assertion of Polydore Vergil that Arthur did not exist. Leland's defense is a tour-de-force of Tudor humanistic analysis in which literary, topographic, linguistic, and archaeological evidence are all considered. Leland examined firsthand many Arthurian relics no longer extant: the famous leaden cross that was uncovered in the 12th c. from Arthur and Guinevere's original grave, the wax seal at Westminster, and the tomb of Arthur and Guinevere at Glastonbury. Dedicated to Henry VIII, the "Assertio" was the only prose work by Leland to be published in the author's lifetime.

From 1533, Leland held a special commission from Henry VIII "to peruse and dylygentlye to searche all the lybraryes of monasteryes and collegies of thys your noble realme". Leland's examinations of England's libraries took him on long tours of Henry's realm, during which he was able to consult not only manuscripts and printed books, but inscriptions, artifacts, and historical sites. These journeys also allowed him to gather oral evidence in support of his thesis. Coupled with the humanistic education he had received in Cambridge and Paris, the King's commission placed Leland in an unparalleled position to consider the evidence concerning Arthur's historicity.

"The 'Assertio', in its attention to detail and its wide erudition, shows one use to which Leland's years of antiquarian research could be put. Although it is repetitive on occasion and often lacks the polish of his poetical works, it stands in other respects as a model of how Leland might have used his massive compilation of notes in other projects had his sanity held…

"In the final analysis, modern scholarship has not moved far beyond Leland in its approach to the question of Arthur's historical existence. Not every document cited by Leland would be accepted by modern scholars, of course, and not every etymology is accurate. On the other hand, his approach resembles ours… The conclusions reached by most historians in the 20th century may be closer to Polydore Vergil's but the methodology resembles Leland's, and in this sense he is indeed the victor in the battle of the books."(James P. Carley, "Polydore Vergil and John Leland on King Arthur")